Technical Field
The embodiments described herein are related to the field of capacitor design, and more particularly to the implementation of capacitors used for minimizing voltage transients.
Description of the Related Art
In electronic circuits, complex components like microprocessors or Systems-on-a-Chip (SoCs) have fluctuating power demands, so capacitors are placed near these devices to hold supply voltages steady as current demand changes. These so-called “decoupling” or “bypass” capacitors are connected between power and ground and act as local low-impedance voltage sources, able to handle transient currents occurring as the load fluctuates. Capacitors manufactured from aluminum or tantalum electrolytics are one choice for decoupling, due to their low cost and large capacitance. In addition, in some embodiments, the voltage levels of supply voltages utilizing these capacitors are held relatively constant during operation, making electrolytic capacitors a suitable choice.
Demand for smaller portable devices drives miniaturization requirements to demand smaller components. Electrolytic capacitors may not provide the best option as they may not provide the smallest capacitor solution. Additionally, power reduction techniques to improve battery life have led to systems that adjust their supply voltages depending on the level of activity of the devices. Modern portable devices may often subject decoupling capacitors to voltages that step dynamically between multiple levels, such as, for example, 0.8V and 1.8V, at time intervals that may be on the order of milliseconds. Again, electrolytic capacitors may not provide the best option as they may not respond to changing voltage levels as quickly as required. Alternatively, advances in ceramic technology have led to Multi-Layer Ceramic Capacitors (MLCCs) suitable for use as decoupling capacitors that may provide physically small components and that allow for faster changes in voltage levels.
However, the combination of MLCC technology and dynamic supply voltages may create an issue. MLCCs use ceramic dielectric materials (e.g., barium titanate) that may change shape slightly when electric fields across the conductive plates of such capacitors change. These shape changes may result from a variety of physical phenomena including the piezoelectric effect, electrostriction, and Coulomb force, and may cause MLCCs to mechanically vibrate in response to a changing voltage level across terminals of such a capacitor. This vibration may couple through capacitor mounting points to excite mechanical vibrations in a circuit board, which may then cause devices to emit audible noise if voltage level changes occur at a frequency in the audible range.
This audible characteristic of MLCCs (commonly referred to as “capacitor singing”) was first observed in MLCC applications involving AC signal filtering, and may be a cause of a noise emanating from an electronic device. Measurements and calculations show that the physical displacements in the capacitor may be extremely small, such that the capacitor surface moves, perhaps, only a fraction of the width of a single atom. Due to large forces that may be involved, the total mechanical power coupled into the system may, however, be macroscopic and audible to humans. Techniques may, therefore, be needed to reduce or eliminate a characteristic such as this. One approach may be to modify the capacitor mounts to reduce the coupling between the MLCC and the circuit board. Another approach may be to arrange multiple capacitors such that most noise is coupled into non-audible resonance modes of the circuit board. However, both techniques may be hampered by the fact that the physical causes of the vibration are diverse and poorly understood, and that the details of the capacitor shape change are influenced by its internal design and vendor processing details, thereby making mass production of such solutions difficult.
A device is desired which may suitably decouple voltage transients on a supply voltage from circuits dependent upon the supply voltage. The desired device should also be of a small form factor for use in portable devices and be resistant to the capacitor singing characteristic just described. Systems and methods for a low-noise capacitive device are presented below.